1. Field of the Invention
The present invention relates to a silicone gel composition which generates a silicone gel cured product that suffers no oil bleeding over time.
2. Description of the Prior Art
Silicone gel compositions are addition reaction curable organopolysiloxane compositions comprising an organohydrogenpolysiloxane containing hydrogen atoms bonded to silicon atoms (namely, SiH groups), an organopolysiloxane containing alkenyl groups such as vinyl groups bonded to silicon atoms, and a platinum-based catalyst, and these compositions generate a cured product via an addition reaction of the silicon atom-bonded hydrogen atoms to the alkenyl groups. Silicone gel cured products produced by heating and curing silicone gel compositions display excellent levels of heat resistance, weather resistance, oil resistance, cold resistance, and electrical insulation, and also display low elastic modulus values and low levels of stress, and as a result, they are widely used in the protection of electronic components such as vehicle-mounted electronic components and household electronic appliance components. The low elastic modulus and low stress level that represent characteristic features of a silicon gel cured product are not found in other elastomer products, but these features are realized by reducing the quantity of the organohydrogenpolysiloxane within the composition, or blending an unreactive non-functional organopolysiloxane oil into the composition, thereby reducing the cross-linking density within the cured product. As a result, the silicon gel cured product contains free oils such as unreacted organopolysiloxane and non-cross-linked oil components. If a silicone gel cured product containing this type of free oil is used for protecting an electronic component, then over time the free oil within the silicone gel cured product tends to bleed out, and depending on the material and design of the electronic component casing, this free oil can leak out through fine gaps. In recent years, instances of such leakage causing contamination of electronic components such as substrates, circuits, wiring, and terminals have been steadily increasing.
A number of methods have been proposed for reducing this type of oil bleeding, including (1) a method of reducing the free oil content by increasing the cross-linking density, (2) a method in which a filler is mixed into the composition, and (3) a method of slowing the movement of the free oil component within the cured product by mixing in an extremely high molecular weight oil to inhibit movement. However, the method (1) risks losing the characteristic low stress property of the silicone gel cured product, and the methods (2) and (3), although reducing the speed with which the free oil bleeds out, do not provide a fundamental resolution of the problem.
Accordingly, the development of a silicone gel composition capable of generating a silicone gel cured product that retains the above characteristic features of silicone gel cured products, while suffering no oil bleeding over time, has been keenly sought.